Update metadata fixup

[danielballan]

An xfail-ed tests caused us to miss a regression. When the MongoAdapter is configured with an access policy (authz_shim), updating metadata raised an error. This PR includes a unit test that reproduced the error:

FAILED databroker/tests/test_broker.py::test_update[mongo-tiled-False] - NotImplementedError: Updating access_blob on MongoDB-backed data is not supported.
FAILED databroker/tests/test_broker.py::test_update[mongo-tiled-True] - NotImplementedError: Updating access_blob on MongoDB-backed data is not supported.

and then a commit with a fix.

It also fixes the confusing mistake within the test itself, which seems to be the that caused us to xfail this test in the first place.

[danielballan]

Test failures...

FAILED databroker/tests/test_broker.py::test_fill_and_multiple_streams[sqlite-tiled-False] - KeyError: 'detfs2'
FAILED databroker/tests/test_broker.py::test_fill_and_multiple_streams[sqlite-tiled-True] - KeyError: 'detfs2'
= 2 failed, 765 passed, 92 skipped, 8 deselected, 47 xfailed, 26 xpassed, 60 warnings in 557.56s (0:09:17) =

are due to a known regression in bluesky, fixed in bluesky/bluesky#1942 but not yet released.

[maffettone]

Looks like the failures in CI are not related to this newly included test.